Chemical switch for detection of chemical components

ABSTRACT

A chemical switch device comprising a film which irreversibly reacts upon exposure to specific chemical components in the environment under the conditions of measurement. The reactions can lead to large changes in the physical and chemical properties of the film which are measurable electrically, optically or by other methods.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 8/456,389,filed Jun. 1, 1995, now pending, which is a divisional application ofSer. No. 08/031,610, filed Mar. 15, 1993, now issued as U.S. Pat. No.5,466,605 as of Nov. 14, 1995.

BACKGROUND OF THE INVENTION

The present invention relates generally to a device and method fordetection of specific chemical components in an environment containingmany distinct chemical species. More particularly, the present inventionrelates to a chemical switch device comprising a film which irreversiblyreacts upon exposure to specific chemical components in the environmentunder the conditions of measurement. The reactions can lead to largechanges in the physical and chemical properties of the film which aremeasurable electrically, optically or by other methods. The term"conditions of measurement" is intended to mean any environmentalconditions under which a reacted or unreacted state of the device can bedetermined. The chemical switches are "yes-no," intrinsically binarydevices that can be miniaturized, mass produced and directlyincorporated into digital electronic circuits.

Typically, electrical switches, such as fuses, are used to provide abreak in an electrical circuit to prevent an electrical overload.Conventional irreversible switches are thermal fuses which fail byphysical breakage of the electrical path due to resistive overheating.In the electrical embodiment of the present invention, an irreversiblechemical switch can exhibit a large change in physical or chemicalproperties upon reaction of the switch material with a specific chemicalcomponent. The apparent physical or chemical change can manifest itselfas a measurable increase or decrease of resistance due to anirreversible change in the switch material from a conductor or resistorto a resistor or conductor, respectively, upon reaction with a specificchemical component. The change is irreversible under the conditions ofmeasurement, much like a conventional electrical fuse, except that theresistance of the irreversible chemical switch can either increase ordecrease upon exposure to specific chemical components. Furthermore,other properties of the device, such as optical or thermal properties,can be used to monitor the extent of the irreversible reactions. Thepresent invention also provides methods for detection of individualchemical components, e.g., hazardous gases, in an environment using anirreversible chemical switch.

The concept of an irreversible chemical switch, based upon irreversiblechemical reactions, is believed to be heretofore unknown. Incontradistinction to electrical fuses, which fail by thermal breakage ofa conductive element, electrical irreversible chemical switches failupon selective reaction of a conductive or resistive material with achemical species; with failure being indicated by an increase ordecrease in resistivity of the conductive or resistive material. Thereaction between the conductive or resistive material and the chemicalspecies causes an irreversible phase change in the conductive orresistive material as the chemical species to be detected forms a newphase, such as an alloy, amalgam or a corrosion product. The phasechange creates a region or zone, propagated through the bulk of thechemical switch, which causes an abrupt change in the electricalresistance, electrical conductivity or other properties of the switchmaterial. Optimally, the rate of change in electrical or otherproperties is rapid and has a sufficient magnitude to provide a reliableand measurable indication of the presence of specific chemicalcomponents.

An important aspect of the invention is the incorporation of highlydurable materials as a selectively reactive element for the conductiveor resistive material. These materials should be capable of beingengineered in thin or thick-film form as the switch materials. Forexample, noble metals, either in substantially pure form, or as alloyswith other noble metals, are very robust, being highly chemically inert,yet can be engineered for specific chemical reactivity. It is known fromthe prior art, however, that some noble metals undergo reversiblesurface reactions with certain chemicals when heated. Noble metal thinfilms (films of less than approximately 10,000 Å thickness), operatingon the basis of surface reactions, have been used as chemical sensorsfor a limited number of chemical species. Another large class ofchemical sensors, metal oxide semiconductor materials, are typicallyheated to between 300° C. and 1000° C. to facilitate adsorption anddesorption of the chemical species on the semiconductor material.Changes in resistivity of the semiconductor material are measured todetermine the presence or absence of the chemical species.

Gold thin films have been used to detect the presence of mercury vapor.McNerney, J. J., et al., Mercury Detection by Means of Thin Gold Films,Science 178:611-612 (1972) disclosed detection of mercury vapor bylinear changes in resistivity in gold thin films having thicknesses of75 Å and 400 Å, with sheet resistivities of 2 to 10 ohms, respectively.McNerney, et al. suggest that the effects of adsorbed mercury atoms onthe conductivity of gold films is a surface effect rather than a bulkalloy effect. U.S. Pat. No. 3,714,562 to McNerney, issued in 1973,(hereinafter "McNerney '562") disclosed that thin film gold layers,having a film thickness of between 75 and 1,000 Å, preferably between 75and 300 Å, undergo resistivity changes upon exposure to mercury vapor.The patent contemplates that other thin film metals may be used todetect the presence of other chemicals to which the metal has a chemicalaffinity. For example, the patent teaches that silver may be used todetect iodine.

The McNerney '562 patent further teaches that if thicker metal films areused, the resistance change becomes masked by the properties of the bulkmaterial. It is taught that the thin films referred to have a mean freepath for electrons which is significantly reduced when a contaminantchemical species is adsorbed onto the film. It was found that uponexposure to mercury vapor, the gold thin film exhibited immediateincreases in resistance. Over time, the rate of change in resistanceincreased slowly, which was believed due to amalgamation of mercury withthe gold; a reaction that can be reversed by heating the gold.

The sensor described in the McNerney '562 patent consists of a glassplate substrate on which a thin layer of gold has been deposited. Thegold layer is configured to provide a reasonably large surface area ofgold and a reasonably long resistance path between electrical terminals.

McNerney '562 recognized that the resistance change in the molecularthickness thin film is a function of concentration of the vaporsadsorbed. The change in resistivity is due to adsorption of the chemicalonto the metal layer. Because the chemical is adsorbed onto the metallayer, without reaction between the gold metal and the chemical, theadsorption is reversible by heating. (Col. 5, L. 13-15). Thereversibility of the adsorption is a key difference between McNerney'562 and the present invention, which provides for an irreversiblereaction between the switch material and the chemical to be detected.The present invention is also distinguished by the use of switchmaterials which can cause resistance to decrease, rather than increase,upon exposure to specific chemical components.

Justi, et al., U.S. Pat. No. 3,973,192, disclosed a device for providingan early detection of aerosol products of combustion originating atleast partly from a polyvinyl chloride substance. The method consists ofmeasuring the electrical resistance of a thin magnesium foil arranged tobe exposed to the aerosol products of combustion. The magnesium foil isprovided with a relatively deep corrosion layer of magnesium dichlorideto accelerate the corrosion of the magnesium foil upon exposure to theaerosol products of combustion. This method is based on the aerosolproducts of combustion, which are formed on heating polyvinyl chloridesubstances to above 100° C. in a moist stream containing hydrochloricacid that can rapidly corrode the magnesium foil. Although this reactionis irreversible, magnesium is a rather reactive alkali earth metal, nota noble metal, and, therefore, cannot reactive selectively or functionas a selective sensor. For example, magnesium will readily react withsteam to form flammable hydrogen.

Takahama, et al., U.S. Pat. No. 4,224,280, disclosed a device fordetecting carbon monoxide which exhibits a stepwise change in filmcurrent over a pre-selected range. The Takahama et al. device employs aplurality of semiconductor films. Three embodiments of the device aredisclosed. A first embodiment consists of a stannic oxide (SnO₂) filmformed on an insulating layer, with a second film layer of predominatelyplatinum (Pt) formed on the first layer of stannic oxide. A secondembodiment is identical, except that gold (Au) is added into theplatinum layer in a gold-platinum alloy. The second layer is depositedwith an average film thickness of 0.3 to 30 platinum atom layers, andthe amount of gold ranges to 50 atomic percent of the platinum. A thirdembodiment contemplates that an electron donor of either antimony (Sb)or bismuth (Bi) is added to the first film layer, and an intermediatelayer of stannic oxide having an electron acceptor selected fromplatinum, aluminum and boron is formed between the first and secondfilms. The insulating film is silicon oxide (SiO₂). Electrodes connectedto lead wires are used to provide a current in the device. A stepwisechange in current results from exposure of the device to an atmospherecontaining carbon monoxide. In this invention, a film of platinum orplatinum and gold, having an atomic thickness that is narrow enough suchthat the film does not show a metallic, electrical conductivity, isformed on a film which essentially contains stannic oxide. (Col. 10,lines 31-36). Col. 6, lines 54-65 suggest that use of a gold secondlayer, i.e., one which is 100% gold, did not yield the characteristicstepwise current change. The use of gold as the second layer is,therefore, not suggested by the reference. Furthermore, FIGS. 7A, 7B and8 of this patent show that the sensors do not show a very large responseto carbon monoxide and that this response strongly depends on theoperating temperatures of the device, which must be above 150° C. Theinventors admit not to understanding the theoretical basis for theoperation of this device. (Col. 6, lines 60-65). Although the inventorsdo not comment explicitly on the reversibility of their sensors, theresponse of heated stannic oxide devices is normally reversible.

U.S. Pat. No. 4,587,104 to Yannopoulos disclosed a gas combustible gassensor which consists of an n-type semiconductor element. Thesemiconductor oxide is bismuth molybdate Bi₂ O₃.3MoO₃. Detection of thecombustible gases is based upon the change of electrical conductivity ofa thick film of the semiconductor oxide. The semiconductor sensor doesnot require a catalyst. The express teaching of the of the Yannopoulospatent is that semiconductor sensors are feasible without the presenceof a noble metal catalyst, such as platinum, palladium and rhodium. Thepresence of a catalyst was previously necessary to yield conductivitychanges in semiconductor oxide films which were large enough to measure.This reference suggests that it is not necessary, or even desirable, toemploy a noble metal element in a gas detector device. The Yannopoulospatent is also based upon the reversible response of this sensor tohydrogen and carbon monoxide.

Komatsu, et al., U.S. Pat. No. 4,592,967, disclosed a gas sensor usingmixed oxides, namely tin oxide, at least one lanthanide oxide, and atleast one of the IVa group element oxides, e.g., titanium (Ti),zirconium (Zr), hafnium (Hf) or Thorium (Th) in a sintered piece coveredwith a porous layer of ceramic. The IVa oxide is present in the range of0.01-20 mol % to keep electric conductance. The gas sensor is constantlyheated to 300°-450° C. to enable rapid adsorption and desorption of thesensed gas on the sintered semiconductor. This type of device wouldclearly be unsuitable for applications where operation at ambienttemperature is required and it is not based upon irreversible reactionsof the sensor material.

Yoshioka, et al., U.S. Pat. No. 4,839,767, describes a device fordetecting internal faults in an insulating gas-charged electricalapparatus. The device consists generally of a substrate, a pair ofelectrodes on the substrate and a thin metal film covering theelectrodes and exposed to the substrate surface. The film producesfluorides with low conductivity upon reaction with a decomposed gasproduced by internal faults of the electrical apparatus. The patentdiscloses that the film may be made of silver deposited on a substrateof Al₂ O₃, with gold electrodes. The device is used to detect faults inapparatus charged with SF₆. SF₆ gas escaping through a fault decomposesto SF₄ or SOF₂, which produces HF upon reaction with trace moisturecontained in the SF₆ gas. The silver film reacts with the HF to produceAgF which increases the resistance of the silver film. The patentdiscloses that an order-of-magnitude change in resistance occurs overmany hours with a thin Ag film having thicknesses of between 100 Å-1000Å. In one instance, there was a very rapid change in resistance, whichrequired heating of the detection element to 80° C. The need for heatingthe detection element to obtain sufficiently rapid increases inresistance renders this arrangement unsuitable for a positiveidentification of chemical species in an ambient environment. Inaddition, the method disclosed by this patent is restricted to use inspecial environments, since silver passivates in the presence of oxygenand is not very selective in its reactivity.

The Koda, et al. patent, U.S. Pat. No. 4,938,928, disclosed a gas sensordesigned for use at elevated temperatures, e.g., 300°-400° C. The deviceconsists of a semiconductor material selected to be specific for the gasto be detected. For example, metal oxide semiconductors of SnO₂, In₂ O₃and Fe₂ O₃ are used to detect combustible and toxic gases; BaSnO₃,LaNiO₃ and NiO are used to detect oxygen; and ceramics such as MgCr₂ O₃or TiO₂ are useful for detecting humidity. Noble metals are used for theheat generating members to heat the semiconductor material to facilitateadsorption and desorption of chemical species onto the semiconductormaterial, resulting in fluctuations in resistance characteristics of thesemiconductor. Again, this type of device is unsuitable for applicationswhere operation at ambient temperature is required, and it is not basedon irreversible reactions of the sensor material.

Bell, et al., U.S. Pat. No. 5,010,021, and its related U.S. Pat. No.5,087,574, disclosed a method for detecting a fluid component within afluid mixture. The method entails the selective adsorption of thecomponent onto a conductive thin layer of material having a chemicalaffinity for the component, and observing the resulting change ofelectrical resistivity of the layer. The adsorption is reversible byheating to desorb the chemical species from the thin layer. The patentsdisclose the use of ozone to increase the dynamic range of the sensor.

With the exception of McNerney '562, Yoshioka '767 and Bell '021, theprior art references teach the use of non-noble metals and/or oxides asthe conductive fuse element where the metal oxide's conductance changesmeasurably upon the adsorption of the particular chemical species to bedetected. Typically, the surface adsorption and desorption reactionsbetween the metal oxide and the chemical species occurs at elevatedtemperatures in the range of 300° C.-1000° C. The ability of thesemiconductor material to desorb the chemical species is critical to thevarious functions of almost all of the prior art detection devices.

McNerney '562 teaches the use of a thin film of gold as a conductiveelement which becomes resistive upon exposure to mercury vapor. McNerney'562 expressly teaches that there is no reaction between the gold andthe mercury vapor, rather the change in resistivity of the gold layer isdue to an amalgamation of the mercury, which sequesters gold layerelectrons resulting in their unavailability for electrical conduction.While the use of thin layer gold to detect mercury vapor is disclosed,the device and manner of use of the McNerney '562 device is distinctfrom that of the present invention. Specifically, the McNerney '562device is designed to detect and measure minute traces of selectedchemicals, employing a metal non-reactive with the chemical species. Theinert property of gold is utilized to assure that the adsorbed mercuryvapor is capable of desorption upon heating of the detection device. Asa consequence, the device does not act as an irreversible chemicalsensor because the reaction is reversible.

A key difference between the present invention and the detection devicesknown in the art lies in the selective, irreversible bulk reaction of achemical species with the chemical switch material in the presentdevice. With the exception of Justi '192 and Yoshioka '767, the priorart discloses reversible surface reactions which induce electricalfailure, but the present invention utilizes chemical activity within thebulk of the switch member to induce irreversible changes in the materialproperties. It is important, in the functioning of a switch, that thechange be irreversible. Thus, the reversibility of the adsorptionreaction at the surface of the sensors of the prior art sensors renderssuch devices inherently unsuitable for use as irreversible chemicalswitches. Furthermore, the nonselectivity, special atmospheres andoperating conditions required for effective operation of the Justi '192and Yoshioka '767 devices render them unsuitable for use as irreversiblechemical switches to detect specific chemical components in theenvironment.

The device of the present invention is capable of undergoing a bulkreaction of the target chemical species at ambient temperature, asopposed to elevated temperatures required by prior art. This isadvantageous because it has been found desirable to detect chemicalspecies within the environment in which they may exist. For example, ifchlorine gas should leak from a train tank car, a chlorine gas detectormust be capable of operation within the temperature ranges in which thetrain tank car operates.

These devices have the potential to significantly advance emissiondetection and process control. For instance, in the emission detectionarena, they have the unique advantages of being able to pinpoint leaksbecause they can be made sufficiently small to be placed at the sourceof the leak. Additional significant advantages of the inventive devicesinclude low cost, low maintenance and no calibration. These chemicalswitches can find applications ranging from detecting leaks inindividual tanks to tank farms to entire manufacturing facilities.Important chemical species with the potential to be detected includehazardous halogens, ammonia, hydrogen chloride, hydrogen fluoride,hydrogen sulfide and methane.

SUMMARY OF THE INVENTION

Accordingly, it is a principal object of the present invention toprovide an irreversible chemical switch device which is irreversiblyreactive to specific chemical species and exhibits a rapid, large changein electrical, optical, or other measurable property, upon reactionbetween the switch material and a specific chemical species. Thereaction is manifested by a phase change in the switch material that,for example, can disrupt or enhance current flow through the switchmaterial. The reaction is irreversible under the conditions ofmeasurement. The present invention further provides a device whichspecifically detects at least one chemical component in the ambientenvironment. The device may be configured to detect more than onechemical species in a mixture or to specifically detect increasingconcentrations of a particular chemical.

More specifically, the present invention is useful for specificdetection of halogen gases such as chlorine. In accordance with oneembodiment of the invention, the irreversible chemical switch is used todetect the presence of a chlorine gas leak by faulting or creating anelectrically conductive pathway. The irreversible chemical switch hasparticular application for storage tanks and mobile tankers whichroutinely transport millions of gallons of highly toxic chlorine acrossthe nation. The inventive irreversible chemical switch may also beconfigured, by selection of the appropriate noble metal, to detect thepresence of chemicals as a warning device, to detect corrosion inpipelines or aircraft, to monitor vehicular emissions from combustionprocesses, or for any similar purpose where the selective detection of achemical component is required.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view schematically illustrating an irreversible chemicalswitch.

FIG. 2 is a cross-sectional view illustrating an irreversible chemicalswitch.

FIG. 3 is a top view schematically illustrating an irreversible chemicalswitch utilizing an interfacial film to achieve poor adhesion betweenthe switch material and a dielectric substrate.

FIG. 4 is a cross-section schematically illustrating an irreversiblechemical switch utilizing an interfacial film to achieve poor adhesionbetween the switch material and the dielectric substrate.

FIG. 5 is a cross-section schematically illustrating an irreversiblechemical switch utilizing an air gap to achieve poor adhesion betweenthe switch material and the dielectric substrate.

FIG. 6 is a top view schematically illustrating a multi-elementirreversible chemical switch design which improves reliability of thedevice by requiring irreversible reaction of multiple switches in orderto signal the presence of a specific chemical.

FIG. 7 is a top view schematically illustrating an irreversible chemicalswitch array designed to detect the concentration of a specific chemicalby utilizing switches of the same material but of different dimensions,which exhibit measurable property changes at progressively higherconcentrations of the specific chemical.

FIG. 8 is a top view schematically illustrating an irreversible chemicalswitch array designed to detect the presence of more than one chemicalby utilizing switches composed of different materials which areselectively reactive with different chemicals.

DESCRIPTION OF PREFERRED EMBODIMENTS

Certain thin or thick films can be engineered to undergo irreversiblechanges in properties, such as electrical, optical or the like, uponexposure to a sufficiently high concentration of a specific chemicalspecies. In this respect, the film behaves effectively as anirreversible chemical switch for the detection of the desired chemicalcomponents. This film serves as the switch material.

The devices described in this present invention work on the basis ofselective, irreversible chemical reactions between the film and one ormore chemical components to be detected, at defective or reactive sitesin the film. Defective and/or reactive regions in the film can bepresent naturally, such as grain boundaries, or can be introducedartificially, such as particularly thin or narrow regions foraccelerated failure. The reaction sites are not limited to surfacemolecules, but occur throughout the bulk of the switch material as well.

As a target molecule adsorbs and/or absorbs to the switch film at areaction site, a reaction occurs producing reaction products that havedifferent properties relative to the starting switch film. For example,in the case of electrical properties, the reaction product can be eitherinsulative or conductive relative to the starting switch film. As thedegree of reaction increases with increasing concentration, there is anassociated increase in the amount of insulative or conductive reactionproducts formed. Because reactive sites are found throughout the bulk ofthe film, insulative or conductive reaction products are integrated intothe bulk, thus decreasing or increasing conductivity, respectively. Forexample, after a sufficient amount of insulative reaction products areformed, current flow is substantially decreased. This decrease can bedetected and used as a switch to fault a circuit, or to initiate awarning or other sequence of events to maintain control of a dangerouschemical. Unlike an electrical fuse that fails due to physical breakage,the irreversible chemical switch undergoes a change in composition uponexposure to specific chemical components.

In order to detect a specific chemical component with a single switch itmust be manufactured of a material that is highly selective to thedesired component, yet substantially inert to all other chemicals thatmay be found in the ambient mixture. For example, for detection of toxicand corrosive chlorine, thin films of gold and gold alloyed with othernoble metals have been found to be ideal switch materials. Except forsurface reactions, which do not play a substantial role in the presentinvention, gold is inert to all of the common gases found in the workplace and environment of the general public. However, gold does reactreadily with only the halogens, such as chlorine, to form bulk halides,such as gold chloride. Thus, gold can be used to accurately detect thepresence of chlorine and other gold halide-forming halogens.

Irreversible chemical switch devices can be used not only as earlywarning devices and process control devices, but can signal long-termdegradation of construction materials. For example, to detect long-termdegradation or corrosion of a pipe carrying natural gas containingcorrosive hydrogen sulfide, the switch material can be designed to failafter long-term exposure to the natural gas. This design is achieved bymanufacturing the switch of a material identical to or simulating theinterior pipe material. The device is then exposed to the natural gasand upon failure, indicates that the bulk pipe materials have degraded,although not yet to a failure point. For this important application, thedetection is rapid in the sense that the chemical switch is engineeredto activate well before the pipe materials have experienced severecorrosion. Such application of an irreversible chemical switch forreliable detection of pipe corrosion or degradation before failure hasimportant benefits for waste, contamination and safety issues. Thereaction mechanism at the reaction sites is dependent on the materialused for the switch and the reactant chemical species, but in generalcan be characterized as a phase change within the bulk of the switchmaterial due to an irreversible chemical reaction, such as alloying orcompound formation. For electrical resistance measurements, these bulkreactions occur to produce insulative or conductive products byirreversible reaction mechanisms. Irreversible reactions involvingchanges in electrical properties offer a very large dynamic range,approaching changes as large as 10²⁰. Such large electrical changes canbe used to differentiate the presence of chemical species that causebulk reactions from those that simply adsorb onto the surface.

Because the device fuse action depends on the number of reactions whichcreate new products, the device reactivity can be crafted by carefulengineering of the properties of the bulk material. For example, if thefilm is made thicker, it takes more time for the irreversible reactionsto permeate the bulk of the film. Consequently, it takes a higherconcentration of chemical species to generate the same response in agiven time. The number of reaction sites within the bulk can also affectthe reactivity of the fuse. For example, the concentration of desiredintrinsic and extrinsic defects, such as bulk defects and grainhomogeneity, which serve as reaction sites, can be controlled by theproper choice of the conditions and materials used to fabricate theswitches.

A variety of electrically insulating fuse substrates can be used tooptimize switch performance, because the substrate influences the grainstructure of the deposited switch material. Substrate materials includeSiO₂ /Si, Al₂ O₃, mica, graphite and polyimide polymers. Thesesubstrates exhibit a broad range of compositions and microstructures tocontrol switch reactivity. SiO₂ has small, compact grain boundaries andis ideal for further device development using semiconductor processingand device technology. A1₂ O₃ can be obtained with a variety of surfaceroughnesses that tend to have a large distribution of grain sizes andassociated voids that promote strong adhesion. Mica is the substrate ofchoice to obtain highly crystalline films having a minimum number ofdefects, and polyimide polymers are flexible, inexpensive materials thatare thermally stable and have been used commercially as substrates forchemical sensors.

Switch reactivity can be further manipulated with chemical modification.Additives can be deposited either simultaneously or serially duringswitch deposition to produce desirable switch performance, such asenhancing and/or controlling sensitivity and/or selectivity. Forexample, gold film reactivity to chlorine may be enhanced by addingsilver, which is more reactive to chlorine, or by adding chromium, whichsuppresses gold's reactivity. Furthermore, switch films can be stackedto produce any desired combination of switch performancecharacteristics.

However, in addition to its effects on the bulk properties of the film,chemical modification can also affect how well the switch materialadheres to the substrate. Adherence to the substrate can affect switchreactivity. Irreversible chemical switches that rely on poorly adheringfilms, such as gold, often require "adhesive" metals to be depositedbetween the substrate and the gold to provide the adhesion necessary fornon-sensing functions such as electrical connections. However, asexplained above, these same metals that are used as adhesives canadversely affect the sensor properties. In the case of gold filmsensors, using chromium as the adhesive layer, repeated or continuousexposure of the fuse to elevated temperatures can cause the chromium tomigrate into the gold film, resulting in loss of sensitivity or failure.This problem can be circumvented by depositing adhesive materials onlyon those regions where they are required, such as electricalconnections, and not on the sensing portion of the device. To eliminateadherence of the sensing portion of the device, the switch material canbe deposited over an air gap so that there is no physical contactbetween the switch material and the substrate.

Alternatively, substantial enhancement in the adhesive forces between afilm and an underlying substrate can be obtained by promoting thepenetration of the film into defects in the substrate. For example, goldadhesion to silica substrates can be improved by annealing thegold/silica structure at temperatures above 600° C. for more than 15minutes, which causes the gold to intrude into the amorphous silicalayer defects.

It should be pointed out that other approaches to measure switchresponse, such as an optical approach using reflected or transmittedlight, need not require the relatively good adhesion needed forelectrical measurement.

By combining several of the factors contributing to the reactivity ofthe switch material, devices of various sensitivities can be created.The following embodiments are intended to serve as examples of theapplication of the invention herein described, and are not to beconsidered limiting.

PREFERRED EMBODIMENT #1

FIG. 1 shows the top view of a basic irreversible chemical switch 10 forthe detection of chlorine. Contact pads 11, connected to a switchmaterial 12, are deposited on a substrate 13. For detection of chlorine,the contact pads 11 and switch material 12 are made of a thin (80-500 Å)and narrow (<100 μ) film of gold. The substrate 13 may be made ofalumina, silica or silicon nitride.

PREFERRED EMBODIMENT #2

FIG. 2 shows a cross-section of an irreversible chemical switch forelectrical resistance measurements having increased functionality bydepositing adhesive material 14 in strategic locations under the contactpads 11. For detection of chlorine, contact pads 11 can be made of goldhaving a thickness of 1,000-10,000 Å and the switch material 12 is madeof a gold film having a thickness of 80-1,000 Å. The switch material 12is less than 100 μ wide and the substrate 13 is selected from the groupconsisting of alumina, silica or silicon nitride. The adhesive material14 is a thin film selected from the group of chromium, titanium oraluminum.

PREFERRED EMBODIMENT #3

FIGS. 3 and 4 illustrate an irreversible chemical switch device forelectrical resistance measurements with the switch material 12 depositedon an interfacial film 16 which has been deposited on a substrate 13.The contact pads 11 exhibit good adhesion to the substrate 13, but theswitch material 12 shows poor adhesion to the interfacial film 16. Fordetection of chlorine contact pads 11 can be made of gold having athickness of 1,000 -10,000 Å and the switch material 12 is made of agold film having a thickness of 80-1,000 Å. The switch material 12 isless than 100 μ wide and the substrate 13 is selected from the groupconsisting of alumina, silica or silicon nitride. The interfacial filmcan be a polymer such as polyimide.

PREFERRED EMBODIMENT #4

FIG. 5 is a cross-section of an irreversible chemical switch device forelectrical resistance measurements based upon suspending the switchmaterial 18 across an air gap 19 on a substrate 20. The switch material18 has no adhesion to the substrate across the air gap 19 and, hence,provides an optimum arrangement for rapid device failure due toselective chemical reactions. The air gap can be created by depositingthe switch region on a sacrificial region of the substrate that can beremoved after deposition.

PREFERRED EMBODIMENT #5

FIG. 6 illustrates a multi-element irreversible chemical switch designedto achieve improved reliability by responding only to multiple switchelement signals. For electrical resistance measurements, switchmaterials 21 can be of equal width, tied to a common electricalconnecting pad 22 and deposited on a common substrate 23.

PREFERRED EMBODIMENT #6

FIG. 7 illustrates a multi-element irreversible chemical switch designedto detect the concentration of a particular chemical component byexhibiting failure at progressively higher concentrations of thecomponent. A plurality of switch elements 24, made of the same materialbut of different thicknesses, are deposited onto an underlyingsubstrate. The response of each of the plurality of switch elements 24is indicative of the concentration of the particular chemical component.The switch materials 24 are tied to a common electrical connecting pad25 and deposited on a common substrate 26. This concentration-leveldesign can be used to assess the maximum concentration of a chemicalcomponent to which the switch array has been exposed.

PREFERRED EMBODIMENT #7

FIG. 8 illustrates a multi-element irreversible chemical switch designedto detect more than one chemical component within a singleswitch-element array. A plurality of switch elements 27 are composed ofdifferent materials which are discretely or differentially reactive todifferent chemical components. For electrical resistance measurements,each of the plurality of switch elements 27 are tied to a commonelectrical connecting pad 28 and deposited on a common substrate 29.This multi-component design can be used to evaluate the proportion ofthe different constituents in a mixture.

PREFERRED EMBODIMENT #8

A basic irreversible chemical switch is designed for the detection ofchlorine gas. Contact pads, connected to a switch material, aredeposited on an insulating substrate. The contact pads and switchmaterial are made of gold films which are 5,000 Å and 150 Å thick,respectively. The switch material is 100 microns wide, and the substrateis silica or silicon. Device exposure to approximately 1% chlorine (byvolume) in air resulted in a resistance increase of seven orders ofmagnitude in six seconds.

Ultra thin films of about 20 Å-80 Å in thickness can be employed ashighly sensitive, rapid reactive switch elements for extremely lowconcentrations of a chemical, such as chlorine. Conversely, a thick filmof between 1,000 Å to 100,000 Å can be used for switches where a largevolume of high concentrations of a chemical, such as chlorine, may beencountered.

While the invention has been fully described with reference to certainpreferred embodiments thereof, those skilled in the art will understandand appreciate that changes may be made and still fall within the spiritand scope of the present invention. For example, alternative measurementmethods, substrate materials, switch materials and dimensions, switcharrangements, or methods of adhering the switch material to thesubstrate may be employed.

What is claimed is:
 1. A system for detecting the presence of a chemicalconstituent comprising:at least one switch element substantially inertand unreactive to primary constituents in an open ambient environment,said at least one switch element being selectively and irreversiblyreactive only with said chemical constituent to be detected in an openambient environment; and means for measuring an electrical response ofsaid at least one switch element; wherein said electrical response ofsaid at least one switch element is caused by the presence of a chemicalconstituent selected from the group consisting of halogens, ammonia,hydrogen chloride, hydrogen fluoride, hydrogen sulfide and methane. 2.The system of claim 1, wherein said at least one switch elementcomprises a noble metal.
 3. A system for detecting the presence of achemical constituent comprising:at least one switch elementsubstantially inert and unreactive to primary constituents in an openambient environment, said at least one switch element being selectivelyand irreversibly reactive only with said chemical constituent to bedetected in an open ambient environment; and means for measuring anelectrical response of said at least one switch element; wherein said atleast one switch element comprises a film with a thickness between 1,000Å and 10,000 Å deposited on a substrate.
 4. The system of claim 3,wherein said at least one switch clement comprises a noble metal.
 5. Asystem for detecting the presence of a chemical constituent comprising:aplurality of switch elements substantially inert and unreactive toprimary constituents in an open ambient environment, said switchelements being selectively and irreversibly reactive only with saidchemical constituent to be detected in an open ambient environment; andmeans for measuring an electrical response of said switch elements;wherein at least two of said switch elements have different thicknessesfor measuring different concentrations of said chemical constituent. 6.The system of claim 5, wherein each said switch element comprises anoble metal.
 7. A system for detecting the presence of a plurality ofchemical constituents comprising:a plurality of switch elementssubstantially inert and unreactive to primary constituents in an openambient environment, wherein said switch element being selectively andirreversibly reactive only with one of said chemical constituents to bedetected in an open ambient environment; and means for measuring anelectrical response of said switch elements; wherein at least two ofsaid switch elements are composed of different material.
 8. The systemof claim 7, wherein each said switch element comprises a different noblemetal.
 9. A system for detecting the presence of a chemical constituentcomprising:at least one switch means substantially inert and unreactiveto primary constituents in an open ambient environment, said at leastone switch means being selectively and irreversibly reactive only withsaid chemical constituent to be detected in an open ambient environment,wherein said chemical reaction yields a phase change in the bulk of theswitch means thereby measurably and irreversibly altering physical andchemical properties of the switch means under the conditions of thechemical reaction; and means for measuring a response of said at leastone switch means; wherein said response of said at least one switchmeans is caused by the presence of a chemical constituent selected fromthe group consisting of halogens, ammonia, hydrogen chloride, hydrogenfluoride, hydrogen sulfide and methane.
 10. The switch of claim 9,wherein said altered physical property is a decrease in said switchmeans' resistance.
 11. The system of claim 9, wherein said at least oneswitch means comprises a film deposited on a substrate.
 12. The systemof claim 9, wherein said measured response is an electrical response.13. A system for detecting the presence of a chemical constituentcomprising:at least one switch means substantially inert and unreactiveto primary constituents in an open ambient environment, said at leastone switch means being selectively and irreversibly reactive only withsaid chemical constituent to be detected in an open ambient environment,wherein said chemical reaction yields a phase change in the bulk of theswitch means thereby measurably and irreversibly altering physical andchemical properties of the switch means under the conditions of thechemical reaction, and means for measuring a response of said at leastone switch means; wherein said at least one switch means comprises afilm with a thickness between 1,000 Å and 10,000 Å deposited on asubstrate.
 14. The system of claim 13, wherein said at least one switchmeans comprises a film deposited on a substrate.
 15. The system of claim13, wherein said measured response is an electrical response.
 16. Asystem for detecting the presence of a chemical constituent comprising:aplurality of switch means substantially inert and unreactive to primaryconstituents in an open ambient environment, each said switch meansbeing selectively and irreversibly reactive only with said chemicalconstituent to be detected in an open ambient environment, wherein saidchemical reaction yields a phase change in the bulk of the switch meansthereby measurably and irreversibly altering physical and chemicalproperties of the switch means under the conditions of the chemicalreaction; and means for measuring a response of said switch means;wherein at least two of said switch means have different thicknesses formeasuring different concentrations of said chemical constituent.
 17. Thesystem of claim 16, wherein each said switch means comprises a filmdeposited on a substrate.
 18. The system of claim 16, wherein saidmeasured response is an electrical response.
 19. The switch of claim 16,wherein said altered physical property is a decrease in said switchmeans' resistance.
 20. A system for detecting the presence of aplurality of chemical constituents comprising:a plurality of switchmeans substantially inert and unreactive to primary constituents in anopen ambient environment, wherein said switch means being selectivelyand irreversibly reactive only with one of said chemical constituents tobe detected in an open ambient environment, wherein said chemicalreaction yields a phase change in the bulk of the switch means therebymeasurably and irreversibly altering physical and chemical properties ofthe switch means under the conditions of the chemical reaction; andmeans for measuring a response of said switch means; wherein at leasttwo of said switch means are composed of different material.
 21. Thesystem of claim 20, wherein each said switch means comprises a filmdeposited on a substrate.
 22. The system of claim 20, wherein saidmeasured response is an electrical response.